There are many applications in semiconductor manufacturing where tight control of temperature, relative humidity, and particles of a process gas such as air becomes critical. See Chang & Sze, ULSI Technology (1996), which is hereby incorporated by reference.
Foundries, including metrology, lithography, and track areas where photoresist and developer is applied to silicon wafers, may require air having a relative humidity of 45%±0.5% at 24±0.1 C, while other cleanroom areas may require a relative humidity of 45%±5% at 24±0.5 C. For example, the humidity of air delivered to a photoresist spin station must be controlled because it affects the evaporation rate of solvent of the resist. To avoid particle contamination, the process areas may also require Class 1 (at 0.12 microns) while the surrounding areas may only require Class 1000 (at 0.3 microns). The velocity of air is in laminar flow region such as 0.35 m/sec.
Foundries must control other parameters affecting the quality of air, including hydrocarbons and other external contaminants, out-gassing of materials contacting the air, the pressure of the air, and the electrostatic discharge of the system delivering the air to the process.
Because of these requirements, especially for cleanliness, some foundries seek to control the local or mini-environments of critical processes, rather than the entire cleanroom, and transport wafers outside mini-environments in SMIF pods (standard mechanical interfaces). Chang & Sze, at pages 12–13. The mini-environments are cost-effective, but require delivery of precisely controlled gas such as air.
In an air-water system, humidification and dehumidification involves transfer of water between a liquid water phase and a fixed air phase which is insoluble in the water. Heat and mass transfer effects influence one another so that the temperature and relative humidity are coupled together.
The specific humidity is defined as the mass of water vapor carried by a unit mass of dry air. Relative humidity is defined as the ratio of the partial pressure of the water vapor to the vapor pressure of the water at the gas temperature, and is expressed on a percentage basis so that 100 percent relative humidity means saturated air and 0 percent means dry air.
Accurate regulation of temperature and humidity requires use of closed-loop control to compensate for disturbances such as changes in the ambient air conditions. The coupling between temperature and relative humidity presents challenges to the design of the closed-loop controller.
The semiconductor industry has attempted to control the temperature and the relative humidity of process air in three stages: first, a chiller lowers the temperature and humidity of the incoming air below the desired values; second, a heater raises the temperature of the air to a desired value, and, third, a steam source humidifies the air to achieve the required relative humidity. However, boiling water at the steam source is an unstable complex process, and it is difficult to provide controlled boiling that generates the required amount of water vapor to control humidity within the required tolerances ±0.5% RH. Furthermore, the response time of humidity levels resulting from change in the heater power supply is too slow and unpredictable. Because the temperature and relative humidity are coupled and it is difficult to precisely controlling the amount of steam used to humidify the air, this conventional approach has significant drawbacks.